In order to simplify the system analysis and theoretical calculation of Sen Transformer (ST), this paper proposes a three-sequence equivalent circuit model with T-shaped structure of ST. Compared with the traditional multi-winding model, this model has a simple topology and the model parameters can be calculated based on the ST experimental parameters, simplifying modeling complexity. In view of the decrease in core ST zero-sequence excitation impedance, the accuracy of the model will be reduced. A more accurate core ST zero-sequence equivalent circuit is proposed to improve the universality of the model. Using the proposed ST equivalent circuit, the influence of ST equivalent impedance under load on the system is analyzed, and the phase shift range of ST and the calculation method of its maximum voltage are given. Based on PSCAD/EMTDC, the steady-state power flow and single-phase short-circuit conditions of ST were simulated and compared with theoretical calculations. The results show that the maximum relative error of ST output current does not exceed 0.1%, and the phase shift angle is consistent with theoretical analysis. The validity of the equivalent model and phase shift range calculation method established in this paper is verified.